The primary goal of the proposed experiments is to identify the nature of Cl minus-dependent glutamate binding to hippocampal membranes and to correlate the binding sites with one of the known physiological glutamate receptors. Chloride-dependent glutamate binding has been found to be two- to fourfold increased after transient exposure of isolated membranes or whole slices to high concentrations of glutamate or tyrosyl-glutamate. The proposed experiments will test three different hypothesis to explain these findings: (i) Modulatory low-affinitiy binding sites for these ligands are converted into or otherwise control the state of high-affinity Cl minus-dependent binding sites. (ii) Glutamate receptors are converted to a desensitized, high-affinity state. (iii) Glutamate becomes sequestered and accumulated in membrane vesicles through a countertransport mechanism. One set of experiments will attempt to answer the question whether glutamate "binding" should be reinterpreted as glutamate sequestration in membrane vesicles. If the traditional interpretation as "binding" is confirmed, electrophysiological and Na+ flux studies in slices will be performed to determine if the induced increase in the number of binding sites correlates with a change in the responsiveness of the cells to synaptic stimulation or to receptor agonists. Radiolabelled phenylalanyl-glutamate will then be synthetized to study the mechanism of binding site induction by dipeptides. A number of questions concerning properties of glutamate binding cannot be satisfactorily resolved without isolating the binding protein. The second experimental goal is to label the binding protein with active site directed reagents, and to purify the labelled protein. The long term objective of this project is to understand the mechanism and the role of receptor regulation in governing synaptic strength in the forebrain.